Move container to marked workspace: refine corner case

This commit should fix "move con to parent" trick (see below) in the
case when con->parent->parent is a workspace.

The trick:

    mark _a, focus parent, focus parent, mark _b,
    [con_mark=_a] move window to mark _b, [con_mark=_a] focus

The trick got broken in commit 626af81232
in order to fix an i3 crash (#2003).  Reverting said commit fixes the
trick.  The crash is caused by the fact that empty workspace isn't
considered a split (checked in src/con.c:1324), so the moved window ends
up as a sibling of the target workspace, not as its child.

testcases/t/243-move-to-mark.t

@@ -361,6 +361,7 @@ does_i3_live;
 ###############################################################################
 # Given 'S' and 'M' where 'M' is a workspace and 'S' is on a different
 # workspace, then 'S' ends up as a tiling container on 'M'.
+# See issue: #2003
 ###############################################################################
 
 fresh_workspace;